Observed enhanced internal tides in winter near the Luzon Strait

Abstract

Seasonal characteristics and nonlinear interaction of internal tides (ITs) near the Luzon Strait in the northeastern South China Sea are investigated using 285 day, in situ observation data. It is found that ITs, which are dominated by the first‐mode wave throughout the year, are the strongest in subsurface layer. Baroclinic incoherent diurnal (semidiurnal) variance accounts for about 85.7% (78.3%) of diurnal (semidiurnal) ITs. The amplitude and seasonal variation of the diurnal ITs are more prominent than those that are semidiurnal, e.g., the largest kinetic energy densities of diurnal and semidiurnal baroclinic tidal currents are 2.81 and 0.83 KJ/m2 in winter, respectively. It is considered that there are two reasons for the significantly enhanced ITs in winter: (1) it may be due to the Kuroshio intrusion, and (2) the enhanced diurnal ITs may be due to the enhanced diurnal barotropic tidal currents, while the enhanced semidiurnal ITs may be caused by the strong nonlinear interaction between diurnal IT constituents O1 and K1 due to their high vertical shears in subsurface layer. Thus, harmonic semidiurnal constituent D2 with a similar frequency of constituent M2 is induced; it subsequently enhances the semidiurnal ITs in this subsurface layer, and the associated energy is carried downward to enhance the semidiurnal ITs in the upper and lower layers.

Cases Study of Nonlinear Interaction Between Near‐Inertial Waves Induced by Typhoon and Diurnal Tides Near the Xisha Islands

“…However, in other cases (e.g., Nameless, ConChan and Mindule in the subsurface layer), fortnightly oscillation was very irregular, and low‐frequency modulation was found, such as in typhoon ConChan, when the period of the spring‐neap tide was nearly one month (the white vertical arrows in Figures c and f indicate the occurrence of the spring tides). This low‐frequency modulation may be due to the enhanced incoherent component of diurnal ITs, which is induced by nonlinear interaction, variable stratification or mesoscale activity (Liu et al, ; Park et al, ). Moreover, results showed that the strong power signals around the near‐inertial period after the passage of each typhoon mainly concentrate in the subsurface layer.…”

“…While strong nonlinear interaction appeared in the subsurface layer, there remains the question as to why the fD 1 currents were not also significantly enhanced in the upper and lower layers, that is, could the fD 1 waves in the subsurface layer after the passage of a typhoon propagate downward like the D 2 waves induced by nonlinear interaction in Liu et al (2015)? The wave number rotary spectra of NICs and fD 1 currents after the passage of a typhoon are shown in Figure 11.…”

“…Near‐inertial waves (NIWs) and internal tides (ITs) are considered two of the most energetic types in the ocean's internal wave spectrum (Alford, ), they often interact with each other (Müeller et al, ). There are primarily two types of interactions that have been identified in the ocean, which are termed “resonant” (weak wave‐wave) (Sun et al, ; Xie et al, ) and “nonresonant” (strong) interactions (Guan et al, ; Liu et al, ). Resonant interaction is primarily due to the resonant relationship (e.g., dispersion, frequency and wave number) among the interaction waves (McComas & Bretherton, ; Zhou & Murtugudde, ).…”

“…However, currently, some observations indicate that the nonresonant interaction also plays an important role in the energy transfer of internal waves, which is found to occur mainly between semidiurnal ITs and NIWs (Mihaly et al, 1998). In the northern South China Sea (SCS), large ITs from the Luzon Strait and wind-induced NIWs are very active, which has attracted some scientists to carry out observational studies (Liu et al, 2015). Meanwhile, the SCS is the largest marginal sea of the western north Pacific and on average, about 10.2 typhoons traverse the SCS per year (Wang et al, 2007).…”

Temporal variability of diapycnal mixing in the northern South China Sea

“…Liu et al . [] also observed enhanced internal tides in winter near the Luzon Strait, which was 1.6 times as large as that in spring, and they considered both Kuroshio intrusion and nonlinear interaction of internal tides contributed to the strong internal tides in winter near the Luzon Strait. The strong internal tides in winter are a likely candidate for the high‐level diapycnal diffusivity in winter.…”